Characterization of surfaces by secondary ion analysis has been an established analytical procedure for more than 20 years [H. J. Liebl, R. F. K. Herzog, Journal of Applied Physics, 34, 2893 (1963) and J. V. P. Long, British Journal of Applied Physics, 16, 1277 (1965)]. Commercial instruments became available in the early 1970's. A typical example of which is the CAMECA-300 (G. Slodzian, Am. Phys., 9. 591 (1964)). The technique shows good sensitivity but not good quantitation. For example, secondary ion sputtering yields vary considerably from element to element, and matrix effects are pronounced and not readily calculable [C. A. Anderson, International Journal Mass Spectroscopy and Ion Physics, 2, 61, (1969) and B. F. Phillips, Journal Vacuum Science and Technology II, 1093 (1974)]. The yield of sputtered neutrals varies considerably less from element to element than secondary ions [Vossen and Kern, Thin Film Processes, Academic Table I, page 15]. A technique to ionize the sputtered neutrals in a systematic manner could lead to an analytical instrument of significant value in quantitative surface analysis. H. Ochsner, W. Gerhard, Physics Letters, 40A, 211 (1972), has shown one approach in which the sample is immersed in a chamber in which a low pressure plasma is generated by a high frequency applied field. The low pressure plasma simultaneously sputters and ionizes the components of the surfaces. An extraction lens system extracts the ions from the plasma for subsequent mass spectrometric analysis. H. Ochsner, Fourth Proceedings of Secondary Ion Mass Spectrometry, Springer, N.Y., 291 (1984), has shown much improved quantitation as compared to analysis of secondary ions alone. A commercial instrument based on Ochsner's work has been introduced as the SNMS by Leybold as model INA3.
A somewhat different approach to evaluation of neutrals is by utilization of a glow discharge. While the energy of the ions with respect to ground potential is low in the case of the high frequency plasma sputtering, the energy of ions made in the glow discharge are typically several thousand electron volts with respect to ground potential. Such high potentials almost dictate that the mass spectrometer be a direction focusing magnetic sector mass spectrometer. A commercial spectrometer utilizing the glow discharge process is the VG9000 manufactured by Vacuum Generators.
A. Benninghoven and A. Muller, Physics Letters A40, 169 (1972), have shown that if low energy, low intensity ion beams or ion beams which have been neutralized impact a surface, then surface removal of neutral species of even labile chemicals occurs. Subsequent ionization and introduction into a mass spectrometer allows for molecular identification, and in many cases, for structural information concerning the parent molecule.
It is an object of this invention to present a single compact ionization source which simultaneously sputters a surface and accepts and conditions both the sputtered neutrals and ions for subsequent mass analysis.
A second object of this invention is the realization of a mass spectrometer system in the form of a probe assembly which can be attached directly to a chamber in which some deposition process is being performed. All of the techniques and existing instruments described above require that a specimen be brought to the instrument. Sample preparation, in some cases extensive, is necessary for all of the techniques and existing instruments. At least one embodiment of this invention described herein is suitable for direct attachment to a production chamber for in-situ analysis and production control.
A third objective is to produce an analytical system simpler and more economical to produce and market than the instruments currently available commercially.